For many decades, aminoalkylenephosphonic acids have been used worldwide at a rate of about 100,000 tonnes of active substance molecule per year. The main fields of use are the detergents and cleaning agents industry and a wide variety of water treatment technologies in which aminoalkylenephosphonic acids and their salts act as multifunctional additives.
Owing to their production, aminoalkylenephosphonic acids are for the most part marketed as aqueous solutions. The solid aminoalkylenephosphonic acids necessary for special applications, for example for use in formulations in powder, lump or paste form, must be produced from the aqueous synthesis solutions in additional process steps. The group of the aminoalkylenephosphonic acids includes a large number of liquid products based on diethylenetriamine penta(methylenephosphonic acid) (DTPMP), which crystallises from aqueous solutions only with difficulty.
The reason for this is the very good complexing power for a large number of metal ions, an excellent stabilisation of water hardness, coupled with a pronounced ability to disperse solid particles and protection of metal surfaces against corrosion. This results in the very wide variety of fields of application of DTPMP, such as, for example, in washing and cleaning processes and as a chelate former in the stabilisation of peroxide bleaches and as an additive for the treatment of drinking water, water for industrial use and oil field water treatment.
Accordingly, there is a great need for DTPMP products which are preferably free of accompanying ions and discolourations and which can be supplied not only as aqueous solutions but also as solids, in order thus to open up a large number of new possible uses to the formulator.
Processes for the production of DTPMP are known and disclosed in detail, for example, in DE 3128755 A1 or EP 1 838 720 B1. However, the products based on DTPMP which are available commercially at present have only inadequate purities and are present only as aqueous products.
DTPMP is obtainable commercially exclusively in aqueous solutions. However, the aqueous products currently offered on the market are not free of accompanying ions and comprise significant amounts of impurities, so that they always have a brownish colour and, in addition, a pronounced characteristic odour. This limits the practical usability of aminoalkylenephosphonic acid, which, from the point of view of application, is extraordinarily flexible.
In this connection there is known, for example from U.S. Pat. No. 4,477,390 A, an exclusively aqueous DTPMP concentrate which is stable to storage at room temperature, wherein the DTPMP is to be kept in solution only in admixture with its lesser substituted representatives (D3A and D4A) and by the addition of high concentrations of at least from 18 to 22 percent by weight of non-oxidising mineral acids (for example HCl).
A large number of attempts are to be found in the literature to improve the product qualities of aqueous solutions of aminoalkylenephosphonic acids or to increase the variety of products. In addition to a wide range of variants for optimising the synthesis, attempts at subsequent purification and the technical production of solids are known.
EP 0411941 B1 discloses in this connection a chemical separation process for the purification of aminomethylenephosphonic acids using an acid-base reaction. After the aminoalkylenephosphonic acid in question has been dissolved in an aqueous base, the aminoalkylenephosphonic acid is recrystallised by the stepwise addition of an acid. The resulting precipitate is then filtered and washed with water. In EP 0411941 B1 it is explicitly mentioned that the claimed process is not suitable for the purification of DTPMP.
EP 724576 B1 discloses a process for the non-alkaline purification of aminoalkylenephosphonic acids. In this process, the crude products, after being suspended in water, are heated to reflux at a neutral or acidic pH. Following the heat treatment, the precipitate is filtered and washed with water. The described process is suitable, for example, for the purification of EDTMP and DOTMP. Because the solubility of DTPMP increases sharply at elevated temperatures of from 60 to 70° C., the process is not suitable for suspending DTPMP in water in order to isolate it under reflux conditions. Recrystallisation by cooling the solution therefore also does not lead to industrially usable amounts.
Typical, commercially available pH-acidic DTPMP products are obtainable exclusively as liquid products because, owing to the impurities they contain as a result of their synthesis, pH-acidic DTPMP solids cannot be manufactured in a storage-stable manner without the aid of stabilisers. Stabilisation of pH-acidic DTPMP liquid products against uncontrolled precipitations is essential. This is effected either by partial neutralisation, at least the trisodium salt or by addition of foreign acids (for example at least 10% by mass HCl).
Granulated sodium salts of DTPMP contain additives from their production, so that the fraction of DTPMP in those solids is significantly less than 40% by mass.
Conventional drying methods such as spray drying or granulation either lead to extremely hygroscopic powders or use technologies in which the active ingredient content of the poorly drying aminoalkylenephosphonic acid DTPMP is lowered by additives, which in turn limits the scope of application of the dry substance. Solid DTPMP products have thus hitherto been unable to establish themselves on the market, as is the case, for example, with the widely used powdered and granular products of the hydroxybisphosphonic acid HEDP.
Since the processes described above for purifying aminoalkylenephosphonic acids containing large amounts of accompanying substances and impurities are not suitable for technical, economic or ecological reasons for purifying and obtaining solid DTPMP, there is therefore a great need for such a process.